Intimacy with the younger one

My #secretconfessed,Male 23. My relationship with my gf was running good for last 2years. But recently in absence of my gf I visited her rented house and I did a mistake. I got physical relationship with her younger sister, she is 19.

Actually we (me + younger sister) used to tease each other whenever we met in the presence of my gf. But that day she (younger sister) was alone. I has been sent by my gf to collect her documents. When I reached her house I met her younger sister and I asked her for the documents of my gf. She started teasing me, I didn't know have by yourself. Then I started teasing with her. While teasing I told her Be afraid of me, I can do anything or everything with you. She replied I will tell my sister (my gf). Then I kept my one step towards her.